There are many applications where there exists a need for transforming rotational motion or torque into another rotational motion and torque. In most cases some kind of planetary and compound planetary gear arrangements may be used for such rotation to rotation arrangements.
Examples of such applications are devices as wind and fluid power mills where it is necessary to transform high torque at low RPM to lower torque with higher RPM. Other examples are motors in cars and trucks, turbines, electric motors and other driving arrangements where it is necessary to convert high RPM and low torque to high torque with low RPM.
At gear arrangements, such as planetary gears, where the transmitted torque is split between several gear components there exists a problem with over determination of the gear arrangement. Such over determination may lead to unbalanced load and uneven wear of the gear components. The present invention is concerned not only with solving problems related to over determination but also to provide shock absorption to reduce transient forces occurring in the gear arrangement when the transmitted forces through the gear arrangement changes drastically. At these types of gear arrangements it is of vital interest that the entire arrangement is durable and has a long and predictable service life. A further important aspect is that volume-weight ratio is as low as possible to the gear-ratio.
The present invention may be used at various applications of the above mentioned type and also in others. An application of certain interest to demonstrate how the present invention can improve the functionality of gear arrangement is the wind power generation industry. In this industry planetary gearboxes play a central part of the driveline from the turbine or rotor blades to the generator for generation of electrical power.
Turbulent winds changes due to large masses in motions, the propeller and generator, give rise to sudden and strong changes of the force direction through the gearbox. This change of direction together with the air gap and stiffness within the gearbox drive train results in shock waves and high overloads on the meshing teeth and in the bearings. Metal contacts between the gear teeth, will contaminate the oil in the gearbox. Optimizing oil qualities to RPM viscosities and cleanness is very important. To extend the lifetime even further it is necessary to create gearboxes where vulnerable parts can be replaced in place with new better optimized components, such as bearings and gears.
Furthermore, as described in NREL Conference paper; NREL/CP-5000-55355; “Dynamic Analysis of Wind Turbine Planetary Gears Using an Extended Harmonic Balance Approach, Preprint dated June 2012, by Y. Guo, J. Keller and R. Parker, transient forces is very common in windmills and might have a negative lifetime factor on this kind of arrangement to achieve even distributions of split torque through the gearboxes.
The cause of the low lifetime of windmill gearboxes is still not fully known though many different kinds of epicyclical gearboxes have been tested. Many windmill manufacturers are today looking for direct driven permanent magnet though they become very large, heavy and expensive using earth metal that is a limited recourse.
A gearbox is a device for power transmission which changes the rotation speed, torque and/or rotation from one rotating shaft to another. This power transfer between the axles is usually made through a collaborative meshing between two cylindrical gears. This means that only two tooth sides at a time with the rolling and sliding movements transmit the current torque. The meshing must, with its two mating tooth flanks, be dimensioned for the torque transmitted between the gears. This may be done by varying the gear diameter, width and meshing. Gear materials and hardness of the tooth flanks as well as the oil that lubricates the gearbox naturally constitute extremely important factors for increased strength and increased resistance to mechanical abrasions. In cylindrical gear boxes an upshifting occurs when a larger diameter driving gear meshes with a gear with smaller diameter driven gear. Correspondingly, downshifting occurs when a smaller diameter driving gear meshes with a larger diameter driven gear. At applications requiring high up- or downshifts, several gear steps can be needed. A first step of e.g. 1:5 could be complemented by a second step of e.g. 1:5, resulting in a total upshift of 1:25. If two meshing tooth flanks must be adapted for large input torques such as e.g. at extraction of wind energy to generate electrical energy, cylindrical gear arrangements can lead to large and heavy gearboxes in relation to its gear ratio. To avoid this problem various different planetary gearboxes have been suggested for providing a compact, energy-dense gear arrangement with high speed ratio, where speed ratio is to be understood as input speed in relation to output speed of the input and output shafts of a gear box.
The planetary gearboxes are built around three types of elements, the planetary gear, planet carrier and the main gear consisting of an outer ring gear or ring wheel and an internal sun gear or sun wheel. With this type of gearbox it is possible to split incoming torques onto several planet gears that are cooperating in transforming torque through the gearbox, between the ring gear and the sun gear.
The planetary gearbox can be made in many ways, but most usually as a single or a compound transmission gearbox. In single planetary gearboxes, a number of single planetary gears transform torque directly between the ring gear and the sun gear.
In compound planetary gear transmissions the torque is transmitted from the ring gear to the sun gear via a number of planetary gear sets, each planetary gear set comprising at least two planetary gears one of which is meshing with the ring gear and one of which is meshing with the sun gear. The planetary gears in each set may, in compound planetary gearboxes, either be in meshing contact with each other or, as in stepped compound planetary gearboxes, be fixed to each other by means of a common shaft. Such compound planetary gear arrangements can generate higher speed ratio than single planetary transmission gear boxes.
The planetary gear box carrier can in some embodiments be connected to the transmission input/output shaft, and thus, with a fixed ring gear, transform torque through its rotating planetary gears to the sun gear and its connection to the output/input shaft. In other examples, the carrier may be fixed to the gearbox which in turn means that the ring gear is connected to the gearbox input/output shaft. The rotating ring wheel conveys thus torque through the fixed planetary carrier with its rotating planetary gears that transform torque to the sun gear that is connected to the output/input shaft.
It is extremely important with split transmission that the transforming of torque is evenly distributed onto the cog flanks that are involved.
With three planetary gears used in a single planetary gear box construction, the sun gear can be centred through the three planetary gear tooth flanks, and by physical and geometric reasons give possibilities to generate even force transmission distribution between the ring gear and sun gear.
In many planetary gear boxes, used for wind turbines, cars etc., three planetary gears in 1-3 steps as single planetary constructions are used to convey incoming primary torque between the ring gear and sun gear.
With three planetary gears in a single transmission gearbox it is possible with internal gearing of the ring wheel, to achieve a gear ratio of about 1:12. At this high gear ratio the sun wheels and its shaft will be disproportional in size and thus usually lower gear ratios are used in practice. With four planetary gears in a single planetary gearbox it may be possible to accomplish a gear ratio of about 1:5.2, with six planetary gears approx. 1:2.5 and with eight planetary gears approx. 1:2, before the planetary gears starts to hit each other.